The present invention relates generally to air bearings and more specifically to devices for shutting off cooling air for air bearings. Air bearings, being more efficient than hydrodynamic bearings, should improve transient response and low speed boost capability of an engine. However, air bearings require cooling air, which is taken from the boost compressor air after the charge air cooler. Therefore, the system efficiency is about the same for air bearings as for hydrodynamic bearings, negating the advantage of the air bearings.
To date, it has not been possible to eliminate the cooling air for the air bearings due to the heat generation at high speeds and the heat flux into the center housing from both the compressor and turbine. However, at low turbo speeds there is not an essential need for the cooling air as the heat generation of the air bearings is low as are the temperatures of the turbine and compressor. An improvement to the turbo system efficiency may be achieved by eliminating the cooling air at low turbo speeds.
U.S. Pat. No. 4,156,342 describes a method of cooling a bearing in a turbine engine where air is bled from an intermediate stage of the compressor turbine and fed, through piping, to the bearing housing. However, the '342 patent does not provide for any means to control the flow of cooling air, especially at low speeds.
U.S. Patent Application Publication 2004/0141836 describes a compressor assembly in which cooling air is diverted from the compressor inlet for cooling an air bearing. A valve controls the flow of air through the air flow path, allowing the flow of cooling air to be shut off when the compressor assembly is not operating. The system does not allow for controlling the flow of cooling air at low speed operation. Moreover, the valve operates via a three force balance, where the outlet pressure from the combustor is fed into a separate chamber and controls the opening and closing of the valve.
As can be seen, there is a need for a device to control the flow of cooling air to an air bearing in a compressor assembly of a turbocharger, particularly a device to shutoff air flow under low speed conditions.